Nerves in higher biological organisms, such as humans or animals, are bundles of long, excitable cells that can extend to meter-order lengths. Cells are referred to as excitable when they are capable of responding to various electric, chemical, optical, or mechanical stimuli by changing their cell transmembrane potential (TMP). A cell's TMP is a measure of the potential difference across the cell's membrane. A TMP can be created due to different concentrations of ions on either side of the membrane. Cells typically maintain lower concentrations of ions inside the cell than the concentration of ions outside the cell to prevent the cell from swelling due to osmosis. Therefore, cells typically have a TMP or are depolarized.
A localized stimulus to an excitable cell, known as an action potential, can affect the cell's TMP. The reduction in TMP causes the cell's membrane to allow sodium ions to rush into the cell, which further reduces the cell's TMP. The reduction of the TMP is known as depolarization. A cell without TMP will swell due to osmosis, therefore, shortly after the sodium inrush the cell expels potassium through the cell membrane. Reducing the potassium concentration inside decreases charge within the cell and increases the TMP. The process of restoring a cell's TMP is known as repolarization.
During the time when the cell is depolarized, it cannot be restimulated by another action potential. This interval is known as the cell's absolute refractory period. The cell's relative refractory period is the interval from partial to complete repolarization. During this time, the cell can be restimulated, but a higher stimulus is required to produce an action potential event, and the response of the excitable cell is lower in magnitude.
Nerve cells are a particular type of excitable cell that are typically characterized by a cell body from which extend dendrites and an axon. The long axon is coated in myelin sheaths and axon terminals extend from the end of the axon. When the nerve cell is stimulated, a depolarization wave travels down the axon to the axon terminals. The axon terminals respond to the depolarization wave by releasing specialized chemicals known as neurotransmitters. The neurotransmitters bind to receptors in the dendrites of adjacent nerve cells and depending on the type of receptor that is activated, will either excite or inhibit the generation of an action potential in the adjacent cell. In this way, signals are passed from one nerve cell to another and enable pulses to be carried along nerve fibers.
Generally, electric currents applied to tissue affect the membranes of excitable cells, causing a depolarizing effect that can lead to a cell action event that depends on the cell type and biological function. Neurostimulation is a term used to describe the artificial excitation or inhibition of nerve cells. Here, small electric currents are applied to excitable tissues of the body such as nerve, muscle, heart, and brain for stimulation and/or control of their functions.
Neurostimulation is thought to be desirable as either a tool for simulating nerve function or for inhibiting the flow of information to the brain (e.g. blocking pain impulses). The ability to selectively stimulate specific nerve fibrils in a complex nerve bundle containing thousands of fibrils is a long-sought capability in biomedical research. Advances in biomedical research have enabled neurostimulators that are currently used in the treatment of many medical disorders including the treatment of pain, epilepsy, Parkinson's disorder, pacing and cardiac arrhythmias, neuralgias, and restoration of lost muscle function, and in brain-machine interfaces.
Neurostimulators are electric pulse generators typically powered by batteries and implanted within the body and which then supply electrical currents to tissues by way of surgically implanted electrical lead wires. Commercial versions of these pulse generators presently have a volume on the order of many cubic centimeters and weight on the order of tens of grams requiring surgery for their implantation, making their use somewhat uncomfortable and cosmetically unattractive. The batteries of these devices need to be either periodically recharged or the whole device explanted and replaced when the batteries wear down.
There has been a trend in the technology to replace large battery powered devices with smaller implanted neurostimulators that do not have batteries but are instead powered inductively through the skin. This reduces the size and bulk of the implanted device. Further reductions in size and bulk are highly desired to reduce the trauma of surgical implantations and to increase patient comfort by reduction in the device's weight.